Which suggests that part of the reason that people have been noticing more contrails since the late 1990s is partly due to the introduction of more efficient high-bypass engines with "cooler and wetter" exhaust, which is more likely to produce contrails, as the exhaust is more likely to freeze before it's fully mixed in the ambient air.

The video is nicely done, and in large part it follows the classic paper on the subject

Experimental Test of the Influence of Propulsion Efficiency on Contrail Formation, Schumann and Busen, 2000.

An experiment was performed in which contrail formation was observed behind two different four engine jet aircraft with different engines flying wing by wing. Photographs document the existence of an altitude range in which the aircraft with high engine efficiency causes contrails whereas the other aircraft with lower engine efficiency causes none. For overall efficiencies of 0.23 and 0.31 and an ambient temperature lapse rate of 12 K/km, the observed altitude difference is 80m (260feet). This value would be larger (200 m, 650 feet) in a standard atmosphere with smaller temperature lapse rate (6.5 K/km). In a standard atmosphere, an increase of overall efficiency from 0.3 to 0.5, which may be reached for future aircraft, would cause contrails at about 700 m (2,300 feet) lower altitude.

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Now this provides a great answer to "what changed in the late 1990s". However I've never really liked to use it, because I've never been clear as to the exact nature or magnitude of the change. For one thing it's obviously been a very gradual change, over a decade or more, and not the sudden change that people claim. So in that sense it's not really different to the gradual rise in air traffic.

For another, the actual amount in increased contrail cover seems to be quite small. The observed difference in formation altitude in the above experiment is just 260 feet, or 650 in ideal conditions. Future increases in engine efficiency might push this to 2,300 feet. But then the paper goes on to say about that:

For an increase of g (efficiency) from 0.3 to 0.5, the threshold formation temperature of contrails for kerosene-driven aircraft increases by 4.2–4.9 K (for 0–100% ambient humidity), implying 650–760 m lower altitude in the standard atmosphere (Fig. 1); the altitude difference increases with RH. The present global mean cover of the Earth by contrails is about 0.1%. If g grows from 0.3 to 0.5 in a future fleet of aircraft, contrail cover is expected to grow by about 20% of its value for otherwise fixed conditions.

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So this very large increase from 0.3 to 0.5 will only increase contrail cover by 20%. This is a lot from a radiative forcing perspective, but does not seem like much in terms of how it might affect the perceptions of individuals. Also it implies that the much smaller increase in efficiencies we've seem with high-bypass engines would result in an even smaller increase in contrail cover.

In summary, while changes in engine efficiency have certainly increased the number of contrails, the increase seems to have been pretty small, simply magnifying slightly the gradual increase in air traffic. While it's a nice point to make, I don't think it's very accurate to blame the chemtrailers perceptions of a sudden change in the late 1990s on the introduction of more efficient engines.

I think the more accurate reason for this perception is simply an individual tipping point, where either they notice the contrails on a particularly favorable day, and hence pay attention to them, or they hear about the "chemtrail" theory, and start looking for them. Unfortunately this reason is much harder to communicate - so it's tempting to use the "new engines" theory. I just don't think it's entirely accurate.

I'll do some more research to try to determine the actual magnitude of the change in contrail cover from engine efficiency, and would appreciate any pointers.

Mick, you could probably write Ulrich Schumann who loks like the go-to guy on the subject. He was helpful to me a decade ago, is familiar with the chemtrail CT, and probably appreciates what you have done with contrailscience. Hermann Mannstein over there at DLR was also helpful.

I think that is part of it, however the higher that jet aircraft can fly, the more efficient they are, so they will try and fly higher. And with RVSM, you can get more aircraft up there than before too since the altitude separation is reduced.

And then we can get into how the regional/commuter airlines, fly primarily with jet aircraft now, compared to flying with turboprops.

I would not so much use the term "local" with that, because many of those regional carriers fly those small jets on routes that are definitely not local in nature. Some of those smaller jets fly routes that used to be flown by mainline jets, and at times where one mainline large jet flew, now 2-3 flights are done by their regional feeders in place of that.

I am not sure "wetter" is correct. I suspect they engineered complete burning a while ago, so the same amount of air and fuel is mixed and burnt to give hot exhaust which includes the same mass fraction of water vapour. The higher efficiency (better use of the energy from combustion) turns up as a higher rate of work being done in exchange for a lower exhaust gas temperature. The RH is higher (ie it is wetter, in a sense) by dint of the lower temperature. This, in turn, allows water condensation to be reached at a not-quite-so-cold environment temperature.

As for altitude, the aim is to fly in the rarest air (least dense = lowest pressure & lowest temperature) for low air resistance and therefore fuel conservation. The altitude is limited above, and the airspeed is limited above and below by the need to keep out of dynamic instability conditions; "coffin corner". http://www.aviationshop.com.au/avfacts/editorial/buffet/default.asp

This is only marginally my area, so please forgive some imprecision in explanation. Tks

It's a shame it can't be simplified to "wetter", as simply saying "colder" begs the question in the reader's mind "what does it matter if it's colder, seeing as it's going to freeze anyway", and then you have to get into mixing, and the difference between RH and RHi, and how liquid water nucleates on aerosols, but ice does not, and then how ice accretes on ice.

What would be a simpler way of explaining the "contrail factor"? It would be nice to explain how different engines have a different contrail factor, but when it's defined as:

The contrail factor is the ratio of water vapor to enthalpy added by combustion to the exhaust plume from an aircraft engine.

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My eyes glaze over. Let's see what other explanations are out there:

As ambient air mixes with exhaust gas, the decrease in a wake parcel’s absolute humidity is directly proportional to the decrease in its temperature (Iribarne and Godson 1981). This means that the wake parcel approaches ambient conditions along a line with a slope equal to the ratio of the water vapor added by engine exhaust to the increase in temperature caused by the heat added to the parcel by the jet engine, as shown in Fig. 1. The slope of this line is defined as the engine contrail factor and is usually expressed in terms of a water vapor mixing ratio per degree. Published contrail factor values vary from as low as 0.0295 g kg−1 K−1 (Pilié and Jiusto 1958) to as high as 0.049 g kg−1 K−1 (Peters 1993). The contrail factor formulation of Busen and Schumann (1995) results in a theoretical minimum value of about 0.028 g kg−1 K−1.

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So you can view it as the amount of water (in grams) produced when raising the temperature of 1 Kg of exhaust gas by 1 degree Kelvin.

That seems initially confusing, as you might mistakenly think that a more efficient engine would produce that heat with less water. But of course the heat is the wasted energy, the less heat, the more thrust, assuming complete combustion.

Explaining the reciprocal would seem more applicable, the rise in temperature caused by the combustion that creates 1g of water. The amount of water is a function of the thrust setting. But then, so it the contrail factor, as the efficiency of an engine is not constant.

Explaining systems of multiple variables is hard. Maybe we should just stick with "modern engines have cooler exhaust, so the water vapor is more likely to freeze before it dissipates"

I thought I would add some further documentation to this thread I ran across. The military did their own internal study named "New Techniques for Contrail Forecasting", with regards to the difference in engine types, essentially to better their forecast abilities back in 1993.

It is a great document with some nice, easy to understand graphs that clearly show how the newer High ByPass Engines will form contrails at higher temperatures, and lower altitudes.